To study the in vivo function of LAIR-1, we generated LAIR-1-/- and LAIR-1flox/flox mice on a C57Bl/6 background. Although LAIR-1-/- mice are healthy and fertile, and have normal longevity in pathogen free environment, they do show certain phenotypic characteristics distinct from wild-type (wt) mice. They develop an increased percentage of splenic B cells and dendritic cells, along with decreased levels of T cells, of which a higher frequency are CD4+CD25+. As the LAIR-1-/- mice age, the splenic T cell population shows a higher frequency of activated T cells and a higher percentage of effector/memory T cells. Since LAIR-1+/+ and LAIR-1-/- T cells traffic with equal proficency to peripheral lymphoid organs, this is probably not the result of abnormal T lymphocyte trafficking. Functional experiments showed that antigen (Ag) specific LAIR-1-/- CD4 T cells proliferate significantly less than Ag specific LAIR-1+/+ CD4 T cells. Although mouse B cells have previously been reported to be negative for LAIR-1 expression, we find that marginal zone B cells express LAIR-1. LAIR-1-/- mice have lower levels of IgG3 and IgG1 in serum and, in response to T-dependent immunization with TNP-OVA, LAIR-1-/- switch less efficiently to IgG2a and IgG2b production, while switching to IgG1 is not affected. Several mouse disease models, including those for experimental autoimmune encephalomyelitis (EAE) and experimental colitis, were utilized to examine the effect of LAIR-1 deficiency in vivo, and no difference in the response of LAIR-1-/- and LAIR-1+/+ mice were observed. Taken together, these observations indicate that LAIR-1 plays a role in shaping the immune response that may be compensated for in vivo by other inhibitory receptors(manuscript in preparation). The immunomodulatory receptor CD300a is expressed on human B cells. Nave B cells express very low levels of this receptor, while memory B cells and plasma blasts/cells express variable levels of CD300a. Germinal center B cells are negative for CD300a expression. Stimulation of naive B cells via BCR and TLR9, along with T cell help, failed to up-regulate CD300a cell surface expression despite the increased expression of the memory marker CD27 and the down-regulation of CD305. TLR9 stimulation alone significantly increased CD300a expression on memory B cells, whereas IL-4 and TGF-beta1 act as negative regulators of CD300a expression on memory B cells. Co-ligation of BCR and CD300a inhibits Ca2+ mobilization and NFAT transcriptional activity evoked by BCR ligation alone. Suppression of CD300a expression by primary B cells with siRNA resulted in increased BCR-mediated proliferation, thereby confirming the inhibitory capacity of CD300a. Finally, we show that of CD300a expression levels are down-regulated in the circulating B cells of HIV-infected patients. Altogether, these data demonstrate a novel mechanism for suppressing the activity of B cells and suggest a potential role for CD300a in the B cell dysfunction observed in HIV induced immunodeficiency (manuscript submitted). Human nave CD4 T cells express low levels of the immunomodulatory receptor CD300a, whereas effector/memory CD4 cells can be either CD300a+ or CD300a-. This suggested that CD300a expression could define a specific subset within the effector/memory CD4 T cell subpopulations. In fact, ex vivo analysis of the IFN-gamma;producing CD4 T cells showed that they are enriched in the CD300a+ subset. Moreover, stimulated CD4 T cells producing TNF-beta;and IL-2 besides IFN-gamma;(polyfunctional) are predominantly CD300a+. In addition to producing markedly higher levels of Th1-associated cytokines, the stimulated CD300a+ CD4 T cells are distinguished by a striking up-regulation of the T-box transcription factor eomesodermin (Eomes), whereas T-bet is up-regulated in both CD300a+ and CD300- activated CD4 T cells to similar levels. The pleiotropic cytokine TGF-beta1 has a determinant role in dictating the development of this Th1 subset, as its presence inhibits the expression of CD300a and down-regulates the expression of Eomes and IFN-gamma. We conclude that CD300a+ human Th1 cells tend to be polyfunctional and after stimulation up-regulate Eomes.